The conduction loss of a diode rectifier contributes significantly to the overall power loss in a power supply, especially in low out-voltage applications. The rectifier conduction loss is the product of its forward-voltage drop, VF, and the forward conduction current IF. FIG. 1(a) shows one kind of rectifier circuit known as a ‘current doubler’. Even when a low forward-voltage drop Schottky diode is used, the voltage drop across D1 or D2 (normally 0.3-0.4V) is still significant compared to the low output voltage (e.g. equal to or less than 5V). If the current through the diode is 1 A, the power loss from diodes is about 0.3 W-0.4 W, which is considerable, compared to the output power, e.g. 5 W.
One solution known in the prior art is ‘synchronous rectification’ (SR), i.e. using a low conduction loss active switch, such as a MOSFET, operating in the III quadrant to replace the diode. A n-channel (n-type) quadrant III MOSFET means that the source terminal is connected to a higher voltage than the drain terminal and current flows from source to drain. A p-channel (p-type) quadrant III MOSFET means that the drain terminal is connected to a higher voltage than the source terminal and current flows from drain to source. The internal resistance of a MOSFET during conduction is normally very low, which consequently reduces the rectifier conduction loss. FIG. 1(b) is a simple schematic of self-driven SR applied to current doubler. The gate drive scheme of the MOSFET is to cross-couple the drive to the input AC voltage.
The prior art describes self-driven SR applied to a Forward rectifier (e.g. reference J. Blanc, S, Inc, Santa Clara, Calif., “Practical application of MOSFET synchronous rectifiers”, Telecommunications Energy Conference, 1991. INTELEC'91, 1991, U.S. Pat. No. 6,038,138, entitled “Self-driven synchronous rectification scheme”, N. Murakami, H Namiki, K Sakakibara, T Yachi, “A Simple and Efficient Synchronous Rectifier for Forward DC-DC Converters”, Applied Power Electronics Conference and Exposition, 1993, U.S. Pat. Nos. 5,625,541 and 5,872,705, entitled “Low loss synchronous rectifier for application to clamped-mode power converters”, U.S. Pat. No. 6,288,920, entitled “Drive compensation circuit for synchronous rectifier and method of operating the same”, W A Tabisz, F C Lee, D Y Chen, “A MOSFET resonant synchronous rectifier for high-frequency DC/DC converters”, Power Electronics Specialists Conference, 1990. PESC'90 . . . , 1990), self-driven SR applied to a Center-tap rectifier (e.g. reference U.S. Pat. No. 6,011,703, entitled “Self-synchronized gate drive for power converter employing self-driven synchronous rectifier and method of operation thereof”, U.S. Pat. No. 6,583,993, entitled “Self-driven synchronous rectification scheme for wide output range”), self-driven SR applied to a Current doubler (e.g. reference U.S. Pat. No. 6,069,799, entitled “Self-synchronized drive circuit for a synchronous rectifier in a clamped-mode power converter”), SR with an auxiliary winding applied to a Forward rectifier (e.g. reference “X. Xie, J C P Liu, F N K Poon, M H Pong, “A novel high frequency current-driven synchronous rectifier applicable to most switching topologies”, Power Electronics, IEEE Transactions on, 2001, P. Alou, J A. Cobos, O. Garcia, R. Prieto, J. Uceda, “A new driving scheme for synchronous rectifiers: single winding self-driven synchronous rectification”, Power Electronics, IEEE Transactions on, 2001, U.S. Pat. No. 6,301,139, entitled “Self-driven synchronous rectifier circuit for non-optimal reset secondary voltage”), SR with an auxiliary winding applied to a Center-tap rectifier (e.g. reference “X. Xie, J C P Liu, F N K Poon, M H Pong, “A novel high frequency current-driven synchronous rectifier applicable to most switching topologies”, Power Electronics, IEEE Transactions on, 2001, P. Alou, J A. Cobos, O. Garcia, R. Prieto, J. Uceda, “A new driving scheme for synchronous rectifiers: single winding self-driven synchronous rectification”, Power Electronics, IEEE Transactions on, 2001, A. Fernandez, J. Sebastian, M M Hernando, P J Villegas and Jorge Garcia, “New self-driven synchronous rectification system for converters with a symmetrically driven transformer”, Industry Applications, IEEE Transactions on, 2005, T. Qian, W. Song, B. Lehman, “Self-Driven Synchronous Rectification Scheme Without Undesired Gate-Voltage Discharge for DC-DC Converters With Symmetrically Driven Transformers”, Power Electronics, IEEE Transactions on, 2008), SR with an auxiliary winding applied to a Current doubler (e.g. reference “X. Xie, J C P Liu, F N K Poon, M H Pong, “A novel high frequency current-driven synchronous rectifier applicable to most switching topologies”, Power Electronics, IEEE Transactions on, 2001, P. Alou, J A. Cobos, O. Garcia, R. Prieto, J. Uceda, “A new driving scheme for synchronous rectifiers: single winding self-driven synchronous rectification”, Power Electronics, IEEE Transactions on, 2001, Y. Panov, M M Jovanovic , “Design and performance evaluation of low-voltage/high-current DC/DC on-board modules”, Applied Power Electronics Conference and Exposition, 1999 . . . , 1999), external controlled SR applied to a Forward rectifier (e.g. reference C. Blake, D. Kinzer, P. Wood, “Synchronous Rectifiers versus Schottky Diodes: A Comparison of the Losses of a Synchronous Rectifier versus the Losses of a Schottky Diode Rectifier”, IEEE Applied Power Electronics Conference (APEC), 1994, M M Jovanovic, M T Zhang, F C Lee, “Evaluation of synchronous-rectification efficiency improvement limits in forward converters”, Industrial Electronics, IEEE Transactions on, 1995), external controlled SR applied to a Current doubler (e.g. reference H J Chiu, L W Lin, “A high-efficiency soft-switched AC/DC converter with current-doubler synchronous rectification”, Industrial Electronics, IEEE Transactions on, 2005, U.S. Pat. No. 6,240,318, entitled “Transcutaneous energy transmission system with full wave Class E rectifier”) and external controlled SR applied to a Flyback rectifier (e.g. reference M T Zhang, M M Jovanovic, F C Y Lee, “Design considerations and performance evaluations of synchronousrectification in flyback converters”, Power Electronics, IEEE Transactions on, 1998).
In the above examples of the prior art, self-driven SR is the simplest, compared to the auxiliary winding version and the external controlled version, because no extra winding or extra controller is needed. From a review of the prior art, however, it can be seen that to date there has been no successful attempt to provide self-driven full-bridge SR. A full-bridge rectifier is an important rectifier circuit which has wide applications. A typical single-phase full-bridge rectifier is shown in FIG. 2. The AC input can be a current source or a voltage source. In the first half cycle as shown in FIG. 2(a), current flows through the input, diode D1, the load and diode D4, which is called a current loop. When current direction reverses, diode D1 and D4 turn off automatically. Current then flows through the input, diode D2, the load and diode D3, as shown in FIG. 2(b), which is another current loop. It must be noted that the automatic turn-off property of a diode is critical to the normal operation of the circuit. A practical self-driven full-bridge SR must therefore have a mechanism of sensing the reverse current for turning off the appropriate switches.
By extending the existing self-driven SR which has been applied to other rectifiers (like the one in FIG. 1(b)), one may derive a straightforward self-driven full-bridge SR circuit, as shown in FIG. 3(a), in which four diodes are replaced by two p-type MOSFETs, M1 and M2, and two n-type MOSFETs, M3 and M4. M1 and M3 are driven by sensing the voltage of point B, while M2 and M4 are driven by sensing the voltage of point A. Such approach is called ‘voltage controlled self-driven’ (VCSD) because the driving signal is coupled to voltage. However, there is a defect in this circuit. As shown in FIG. 3(b), the current loop through M1 and M4 can flow in both directions, because VCSD gate drive cannot detect the reverse current. So does the loop through M2 and M3. Unlike the diodes in FIG. 2, which can automatically turn off when their current reverses, such bidirectional switch current flow can make the commutation fail.
Some prior art has dealt with full-bridge SR with other approaches. U.S. Pat. No. 7,269,038, entitled “VRMs and rectified current sense full-bridge synchronous-rectification integrated with PFC” employs an external controller that is suitable for the application of PFC (Power Factor Correction). Also known in the prior art is “A Rechargeable Battery Circuit and Structure for Compatibility with a Planar Inductive Charging Platform (U.S. patent application Ser. No. 11/234,045), but that proposal needs the help of a resonant capacitor at the secondary winding creating a sinusoidal voltage waveform and a smoothing inductor at the output to enhance the turn-off timing. But the passive capacitor and inductor are large in size and this inevitably creates a large dead-time between the driving pulses that adversely affects the duration of power transfer in one cycle. This kind of approach has a major limitation. Eventually it changes its tactic to use an external digital PLL controlled SR to achieve its miniature. It is still not a self-driven full-bridge SR.